1. Field of the Invention
This invention generally relates to an apparatus and a method for measuring the acoustic properties, such as the resonant tuning, of a membranophone, such as a drumhead of a drum. More specifically, this invention relates to an apparatus and a method for measuring or monitoring the sounds made by a drum at the time the tension of a drumhead of the drum is being adjusted.
2. Description of Related Art
Musical performance drums have one or more beatable membranes which are called drumheads. The drumhead is mounted on the upper edge, and optionally the lower edge, of the generally cylindrical drum body. A drumhead tightening hoop or ring is typically placed on the outer peripheral edge of a drumhead around the drum body. Typically, a plurality of tightening bolts are screwed to respective lug nuts which have been arranged at spaced intervals around the outer periphery of the drum body. Usually, the tightening bolts and lug nuts are equally spaced around the periphery of the drum body. The drumhead tightening hoop may be compressively tightened over the drum body using the tightening bolts and lug nuts to fix the drumhead to the drum body. A tuning key or wrench tightens the drumhead by selectively tightening each tightening bolt and its lug nut. Generally, the drumhead is evenly stretched over the entire drum body edge, such as the upper and/or lower edge, prior to being used, such as in a musical performance.
Obtaining a relatively specific or precise amount of stretching of the drumhead is important for the performance of the drum in general. For example, stretching a drumhead too tightly may lead to durability problems. Such durability problems may be attributed to the tension of the drumhead exceeding the range of elastic deformation of the material from which the drumhead is made. For example, many drumheads include a polyester film. By excessively tightening the drumhead, the range of elastic deformation may be exceeded, or the tension of the drumhead may be greater than the material's (e.g., polyester film) yield point, thereby causing deformation or rupture of the drumhead. Alternatively, any drumhead attached with inadequate tension may not maintain continuous contact with the edge of the drum body when struck by a beater and thus, the drum will generate non-musical sounds. When the drumhead is tensioned between these extremes (e.g., too tight and too loose) and the drumhead is struck by a beater, the drum typically resonates at multiple frequencies.
Circular drumheads typically have multiple resonances. The first or lower primary frequency involves the entire drumhead and is a product of the average drumhead tension. Influenced by the geometry of a circular drumhead, higher frequency modes typically follow sundry paths across and around the drumhead. Due to flexure of the drumhead tightening hoop, these higher frequency vibratory modes are also often influenced by the local tension associated with each tightening bolt/lug nut pair. If the drumhead is not precisely tensioned at every tightening bolt/lug nut pair, the overtone vibratory modes (e.g., higher frequency modes) will be diffuse when the drumhead is struck, thereby resulting in diminished tonal purity, and reduced sustainability.
In the past, to tune the drumhead and to clear the drumhead of these non-musical overtones, the drumhead was lightly beaten with a stick in the vicinity of each tightening bolt/lug nut pair. The prominent overtone sound which was generated was compared with the sound generated by beating in the vicinity of the other tightening bolt/lug nut pairs so that each tightening bolt/lug nut pair might be adjusted with the tuning key so as to eliminate the difference between the higher and the lower tensioned pairs. An alternate approach was to apply pressure with a finger to the drumhead at or near its center to create a node or point of no vibration. When lightly beaten with a stick at points midway between the center and each tightening bolt/lug nut pair radially in line with that pair, the lower primary resonance was suppressed and the overtone sound related to the tension from that tightening bolt/lug nut pair became more apparent.
Unfortunately, while performing this adjusting operation, a person typically must listen to the sounds generated by striking the drumhead and rely upon short term tonal memory of the frequencies being generated. Tuning success using this method requires accurately striking the drumhead at specific locations with consistent force, while precisely remembering the exact tone generated by previous tests. In addition, minor changes in tension applied to the drumhead to dampen the primary vibration mode in the alternate tuning approach will likely alter the apparent tension of the drumhead and introduce adjustment errors. Tension errors from adjacent tightening bolt/lug nut pairs may also interact and complicate this tuning effort. The skill and patience to complete such adjustments using these methods are typically beyond the ability of many drum owners who routinely must perform with poorly tuned drumheads. To address these problems, mechanical tuning aids have been developed to assist drum owners in this difficult task.
One mechanical aid is disclosed in U.S. Pat. No. 4,287,806. The device disclosed therein involves use of a torque indicator on each tightening bolt to facilitate consistent lug tension. While this can be an improvement over manual tuning, the relationship between tightening bolt torque and drumhead tension can be degraded by thread quality and condition of the tightening bolt and lug nut. In addition, any deviations from flatness in the drumhead tensioning hoop and drum shell knife edge will likely introduce distortions in the tension applied to the drumhead. The success of this approach requires very accurate drum manufacturing tolerances and the fitness of the mechanical hardware.
A second mechanical device is disclosed in U.S. Pat. No. 5,533,405. The device disclosed therein attempts to overcome some of the limitations associated with the device disclosed in U.S. Pat. No. 4,287,806 by applying a known force directly to a point on the drumhead and measuring the deflection of the drumhead. The amount of this deflection may be representative of the mechanical properties of the drumhead material, drumhead material thickness, and/or tension at that location. This is an improvement over U.S. Pat. No. 4,287,806 but still suffers inaccuracies from mechanical or fabrication variations in the drum/drumhead system, and requires precise placement on the drumhead for reliable measurements.
One weakness common to the approaches disclosed in U.S. Pat. Nos. 4,287,806 and 5,533,405 is that the measurement methods are indirect. The methods and devices do not actually measure the vibration of the excited drum/drumhead system but measure mechanical parameters that influence this vibration and rely upon other unmeasured parameters to be ideal or held to very precise manufacturing tolerances. If any of these unmeasured parameters deviate the resultant drum tuning accuracy will deteriorate.
In addition, external frequency references such as tuning forks have also been used in orchestral settings to aid in average tuning of drumheads but do not offer any specific relief from the problem of balancing relative tension of individual tension bolts/lug nut pairs, as discussed herein.